In a motor vehicle, it is known to use an access point unlocking system comprising a sensor for detecting the presence of a user, equipped with an authentication card. For example, such a sensor can thus detect the presence of a hand of the user on the handle of a door so as to unlock the door, or can detect the passing of a foot of the user in front of a sensor arranged beneath the luggage compartment of the vehicle so as to unlock the luggage compartment and open it at least in part.
To do this, the sensor is connected to an electronic control unit (ECU) of the vehicle, to which it sends a presence detection signal enabling it, for example after authentication, to unlock the corresponding access point.
Such a detection sensor is present in the form of a capacitive proximity sensor adapted to detect the presence, for example, of a hand or a foot of a user. Thus, for example, as illustrated in FIG. 1, the user can bring his hand M toward a sensor 3, arranged in a handle 6 of a door of the vehicle, during a movement between a first position 1 distanced from the sensor 3 and a second position 2 in the proximity of the sensor 3 in order to unlock the door. Thus, if a user brings his hand toward a vehicle door, the sensor arranged in the handle of said door makes it possible to detect a presence on account of the modification of the charge of the proximity sensor.
The detection sensor 3 is provided in the form of an electric circuit comprising a detection electrode 4 comprising a detection capacitor Cx connected to a printed circuit 5 comprising a “storage” capacitor Cs. In such a sensor 3, the user, whilst in the proximity of the electrode 4 of the sensor 3, behaves as a second electrode connected to ground, which increases the charge of the detection capacitor Cx. Such an increase is then evaluated via the storage capacitor Cs, as described hereinafter, so as to detect the presence of the user in order to allow the unlocking of the corresponding access point.
A known detection sensor is provided in the form of an electric circuit A, illustrated in FIG. 2, comprising a voltage generator Vcc, an electrode, represented by a detection capacitor Cx, a storage capacitor Cs, a resistor Rc and three bi-position switches S1, S2 and S3. The first switch S1 is arranged between the positive terminal of the voltage generator Vcc and a terminal of the detection capacitor Cx. The second switch S2 is arranged between the same terminal of the detection capacitor Cx and a terminal of the storage capacitor Cs, the other terminal respectively of each of the detection Cx and storage Cs capacitors, as well as the negative terminal of the voltage generator Vcc, being connected to a ground G. The third switch S3 is arranged between the positive terminal of the voltage generator Vcc and a terminal of the resistor Rc, of which the other terminal is connected to the terminal of the storage capacitor Cs, which is in turn connected to the switch S2. In order to detect the presence of a user, the sensor functions alternately in a first “acquisition” phase and a second “measurement” phase.
The acquisition phase comprises a cycle repeated a predetermined number (x) of times so as to charge the storage capacitor Cs at least with a reference charge Cref corresponding to a reference storage voltage Vsref at the terminals of the storage capacitor Cs. Such a sensor is said to be a “low-consumption linear charge-transfer” sensor.
A cycle of the acquisition phase comprises four steps enabling a linear charge transfer between the voltage generator Vcc and the storage capacitor Cs via the detection capacitor Cx. In the initial state of the circuit, the three switches S1, S2 and S3 are open. The third switch S3 remains open during the four steps of the acquisition phase. In a first step C referred to as a “charge” step, the first switch S1 is closed and the second switch S2 is open, which allows the detection capacitor Cx to be charged C by the voltage generator Vcc. In a second step referred to as a “rest” step, as illustrated in FIG. 2, the first switch S1 and the second switch S2 are open simultaneously. In a third step D referred to as a “discharge” step, the first switch S1 is open and the second switch S2 is closed, which allows the detection capacitor Cx to be discharged D into the storage capacitor Cs, that is to say allows induction charging of the storage capacitor Cs via the detection capacitor Cx. Lastly, in a fourth rest step, the first switch S1 and the second switch S2 are again opened simultaneously, as illustrated in FIG. 2.
When the storage capacitor Cs has been charged for a predetermined number x of cycles without a user having come into the proximity of the sensor, that is to say without the detection capacitor Cx having also been charged by the presence of a user in the proximity of the electrode 4 of the sensor 3, the charge of the storage capacitor Cs at the end of the acquisition phase corresponds to the reference charge Cref. In other words, the storage voltage at the terminals of the storage capacitor Cs corresponds to the reference storage voltage Vsref at the end of the acquisition phase.
By contrast, when the presence of a user has allowed the charging of the detection capacitor Cx, the charge of the storage capacitor Cs at the end of the acquisition phase corresponds to the reference charge Cref plus a detection charge Cdet of the user. In this case, the storage voltage Vs at the end of the acquisition phase is greater than the reference storage voltage Vsref.
During the measurement phase, in which the first switch S1 and the second switch S2 are open, the third switch S3 is closed so as to charge the storage capacitor Cs (reference M in FIG. 2), thanks to the charge current Ic passing through the resistor Rc, until the storage voltage Vs reaches a threshold voltage VTH, and the period of time between the moment of closure of the third switch S3 and the moment at which the storage voltage Vs reaches said threshold voltage VTH is then measured.
With the absence of the user in the proximity of the sensor during the acquisition phase, the period of time measured between the moment of closure of the third switch S3 (at which the storage voltage Vs is equal to the reference voltage Vsref) and the moment at which the storage voltage Vs reaches the threshold voltage VTH corresponds to a reference period Tref. In other words, in the absence of detection, the storage voltage Vs reaches the threshold voltage VTH after a reference period Tref. With the presence of a user in the proximity of the sensor 3 during the acquisition phase, the period of time measured between the moment of closure of the third switch S3 (at which the storage voltage Vs is equal to a detection voltage Vdet greater than the reference voltage Vsref) and the moment at which the storage voltage Vs reaches the threshold voltage VTH corresponds to a detection period Tdet, which is shorter than the reference period Tref, which indicates the detection of the presence of a user in the proximity of the electrode 4 of the sensor 3. In other words, during the detection of the presence of a user, the storage voltage Vs reaches the threshold voltage VTH more quickly during the measurement phase, and the detection period Tdet is thus shorter than the reference period Tref. By measuring the detection period Tdet, the presence of a user close to the sensor 3 is thus determined.
If the length of the electrode 4 comprising the detection capacitor Cx is significant, as may be the case with certain types of sensors 3, for example for the opening of a luggage compartment of a motor vehicle, the emissions radiated by the electrode 4 are significantly more substantial than those radiated by a shorter electrode 4, used, for example, in a motor vehicle door handle. Such radiated emissions may cause electromagnetic disturbances over certain electronic systems installed in the vehicle, which presents a disadvantage.
An immediate solution for reducing the level of radiated emissions lies in significantly reducing, in the electric circuit A, the voltage Vcc that generates the emissions radiated along the electrode 4.
A low value of the voltage Vcc, for example divided by ten, makes it possible, however, to charge the storage capacitor Cs by the value of Vcc in a single cycle, but leads to a significant decrease of the sensitivity of the sensor 3, unfortunately affecting the reliability thereof.
In addition, a low value of voltage Vcc limits the charge of the storage capacitor Cs, which may thus be substantially equal to the residual noise contained in the storage capacitor Cs. As a result, the measured detection period Tdet is substantially equal to the reference period, which does not allow reliable detection of the presence of a user and again constitutes a disadvantage.